Tissue products, such as facial tissues, paper towels, bath tissues, napkins, and other similar products, are designed to include several important properties. For example, the products should have good bulk, a soft feel, and should have good strength and durability. Unfortunately, however, when steps are taken to increase one property of the product, other characteristics of the product are often adversely affected.
To achieve the optimum product properties, tissue products are typically formed, at least in part, from pulps containing wood fibers and often a blend of hardwood and softwood fibers to achieve the desired properties. Typically when attempting to optimize surface softness, as is often the case with tissue products, the papermaker will select the fiber furnish based in part on fiber length, aspect ratio and thickness of the fiber cell wall. Unfortunately, the need for softness is balanced by the need for durability. Durability in tissue products may be defined in terms of tensile strength, burst strength and tear strength. Typically tear strength and burst strength have a positive correlation with tensile strength while tensile strength, and thus durability, and softness are inversely related. Thus the paper maker is continuously challenged with the need to balance the need for softness with a need for durability. Unfortunately, tissue paper durability generally decreases as the average fiber length is reduced. Therefore, simply reducing the pulp average fiber length can result in an undesirable trade-off between product softness and product durability.
Besides durability, long fibers also play an important role in overall tissue product softness. While surface softness in tissue products is an important attribute, a second element in the overall softness of a tissue sheet is stiffness. Stiffness can be measured from the tensile slope of stress-strain tensile curve. Generally, a decrease in tensile slope results in lower stiffness, which typically provides better overall softness. However, at a given tensile strength and slope short fibers will display a greater stiffness than long fibers. While not wishing to be bound by theory, it is believed that this behavior is due to the higher number of hydrogen bonds required to produce a product of a given tensile strength with short fibers than with long fibers. Thus, easily collapsible, low coarseness long fibers, such as those provided by Northern softwood kraft (“NSWK”) fibers typically supply the best combination of durability and softness in tissue products when those fibers are used in combination with hardwood kraft fibers, such as Eucalyptus hardwood kraft (“EHWK”) fibers. While NSWK fibers have a higher coarseness than EHWK fibers, their small cell wall thickness relative to lumen diameter combined with their long length makes them the ideal candidate for optimizing durability and softness in tissue.
Unfortunately, supply of NSWK is under significant pressure both economically and environmentally. As such, prices of NSWK have escalated significantly creating a need to find alternatives to optimize softness and strength in tissue products. Another type of softwood fiber is Southern softwood kraft (“SSWK”), which is widely used in fluff pulp containing absorbent products such as diapers, feminine care absorbent products and incontinence products. Unfortunately while not under the same supply and environmental pressures as NSWK, SSWK fibers are generally poorly suited for making soft tissue products. While having long fiber length, the SSWK fibers have too wide a cell wall width and too narrow a lumen diameter and thus create stiffer, harsher feeling products than NSWK.
The tissue papermaker who is able to obtain pulps having a desirable combination of fiber length and coarseness from fiber blends generally regarded as inferior with respect to average fiber properties may reap significant cost savings and/or product improvements. For example, the papermaker may wish to make a tissue paper of superior strength without incurring the usual degradation in softness which accompanies higher strength. Alternatively, the papermaker may wish a higher degree of paper surface bonding to reduce the release of free fibers without suffering the usual decrease in softness which accompanies greater bonding of surface fibers. As such, a need currently exists for a tissue product formed from a fiber that will improve durability without negatively affecting other important product properties, such as softness.
Outside of softwood kraft pulp fibers very few options exist for papermakers when seeking a satisfactory fiber to provide strength without negatively impacting softness. Thus, there remains a need for alternative papermaking fibers that may deliver softness while maintaining satisfactory strength.